I posted a comment about this in a thread I had made about something else, but then I realized I wouldn't get as many replies as I'd like.
Regarding the battle with Jyggalag, I wonder about the implications of a mortal fighting against a Daedric Prince directly like that. Deities in general are untouchable in TES. A mortal cannot defeat one of them in single combat, no matter how strong he is.
That's why the best you could do in the Dragonborn questline was defeating the pawn Miraak, as Hermaeus Mora himself is beyond your reach regardless of your unique powers.
Sure, there is the final battle with Hircine in the bloodmoon DLC, but you don't really face his full power, just a portion of it.
And just to avoid making too many threads, I'd like to add a second question. Considering that Mehrunes Dagon was defeated by a mere fraction of Akatosh's original power (before Creation), aren't Aedra actually far superior to the Daedra?